When an error state is an OOM, make sure that we treat is specially
and do not try to free it.

We use heuristics to make a decent guess at when we can save time and
space by linking object files during a clone. Unfortunately checking the
device id isn't enough, as those would be the same during e.g. a bind-mount,
but the OS still does not allow us to link between mounts of the same
filesystem.

If we fail to perform the links, fall back to copying the contents into
a new file as a last attempt.

It has now become a no-op, so remove the function and all references to
it.

The base refspecs changing can be a cause of confusion as to what is the
current base refspec set and complicate saving the remote's
configuration.

Change `git_remote_add_{fetch,push}()` to update the configuration
instead of an instance.

This finally makes `git_remote_save()` a no-op, it will be removed in a
later commit.

While this will rarely be different from the default, having it in the
remote adds yet another setting it has to keep around and can affect its
behaviour. Move it to the options.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

We want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.

This namespace is about behaving like git's branch command, so let's do
exactly that instead of taking a reflog message.

This override is still available via the reference namespace.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

With opportunistic ref updates, git has introduced the concept of having
base refspecs *and* refspecs that are active for a particular fetch.

Let's start by letting the user override the refspecs for download.